JP3393058B2 - Method of forming rust of steel with excellent corrosion resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming rust on a steel material having excellent corrosion resistance, which is suitable for a structural material such as a bridge which is difficult to maintain and manage and which is used with or without painting. It is about.

[0002]

2. Description of the Related Art For example, steel materials used for bridge structures such as road bridges in salt corrosive environments such as salt water and snow melting salt coming in in mountainous areas and coastal areas have been conventionally coated to improve corrosion resistance. It is used. However, since this coating film always deteriorates with time, it is necessary to maintain and maintain the corrosion resistance by recoating the coating film at regular intervals.

On the other hand, in recent years, a small number of main girder bridges with a small number of main girders represented by two main girder bridges have come to be used in many of these bridges in place of conventional multi-girder bridges. There is. This minor girder bridge can reduce the amount of steel used (steel weight) and the number of bridge pieces compared to the majority girder bridge, has good workability, and has advantages in terms of environmental protection and shortening the construction period.
Further, such a minority main girder bridge is strongly required to minimize the load and cost of maintenance and management after the bridge is installed and to extend the life of the bridge itself.

Therefore, steel materials used for structural materials such as steel towers and buildings, including such minority main girder bridges, etc., can be used unpainted (naked) under the salt corrosion environment. Further, there is a strong demand for a steel material that maintains high corrosion resistance so that even if the coating film is used after being painted and the coating film is deteriorated or destroyed during use, maintenance is not required after installation of the bridge in any case.

Conventionally, in order to improve the corrosion resistance of this kind of steel material, various improvement techniques have been proposed from the side of the steel material which is the base material. For example, as a typical example, P: 0.15% or less or C
There is a weathering steel containing u: 0.2 to 0.6%, Cr: 0.3 to 1.25%, Ni: 0.65% or less. Two types of JIS G 3114 (weather resistant hot rolled steel for welded structures) or JIS G 3125 (high weather resistant rolled steel) are standardized for this weather resistant steel. This weather resistant steel, due to the action of the trace elements, rust generated on the steel surface during use of the steel material becomes a dense stable rust layer (weather resistant rust) having high corrosion resistance typified by bare weather resistance. Has a self-corrosion function. Due to such properties, the weather-resistant steel has been basically used unpainted as a maintenance-free structural material for various structures such as the bridge.

However, under the salt corrosive environment, the stable rust layer, which is a characteristic of weather resistant steel, is less likely to be formed due to the influence of salt. If the stable rust layer is not formed, the corrosion resistance of the weather resistant steel will be significantly reduced. This is because in the corrosive environment with a large amount of salt, the pH in the rust film is particularly lowered as the steel corrodes. That is, when corrosion of steel begins even slightly, first, Fe → Fe ²⁺ ＋ 2e ⁻ , followed by Fe ²⁺ ＋ 2H ₂ O → Fe (OH)
_The reaction of ₂ + 2H ⁺ lowers the pH of the steel surface and also lowers the pH in the rust film and at the interface between the rust film and the steel. Then, once the pH is lowered, the transport number of chlorine ions in the rust film increases in order to maintain electrical neutrality, and concentration of chlorine ions occurs at the interface between the rust film and steel. As a result, a hydrochloric acid atmosphere is formed at this interface portion, which promotes corrosion of steel. At the same time, due to the decrease in pH in the rust film, the solubility of iron ions increases, and a phenomenon that hinders the formation of the stable rust layer, which is the key to the anticorrosion mechanism of corrosion-resistant low-alloy steel such as weather-resistant steel, also occurs. , A corrosion acceleration situation is formed.

Therefore, in order to prevent a decrease in pH in the rust film, a technique has been proposed in which the surface of the weather resistant steel is alkalized to prevent the formation of the accelerated corrosion condition. More specifically, Be, Mg, C that alkalizes the surface of weathering steel.
Oxides (chemical species) such as a, Sr, and Ba are dispersed in steel in advance, and at the same time as the corrosion reaction of the steel, these chemical species act to suppress the decrease in pH of the steel surface. , For example,
It is proposed in Japanese Patent Laid-Open No. 58-25458 and Japanese Patent No. 2572447.

The technique of adding these oxides to prevent the formation of the accelerated corrosion state is certainly effective in suppressing the influence of salt and the like from the outside. However, like the weather resistant steel, it is difficult or limited to form the stable rust layer itself, and in reality, sufficient corrosion resistance is not obtained. In addition, the oxide itself added to the steel is
There is also a concern that characteristics such as weldability and strength may be adversely affected.

Therefore, in order to solve the problem of improving the corrosion resistance of steel materials, various techniques have been proposed for forming the stable rust layer itself by surface treatment of steel materials, not by improving the composition of the steel materials. For example, in Japanese Patent Laid-Open No. 06-93467, the steel surface is coated with rust composed of α-FeOOH containing 0.3 wt% or more of Cr, Cu, P, and Ni, or this rust. Cr, Cu, P, Ni to form
A technique of applying an aqueous solution containing ions to the surface of a steel material is disclosed.

Further, Japanese Unexamined Patent Publication No. 09-125224 discloses a technique of heat-treating steel to coat the steel surface with rust composed of hematite (αFe ₂ O ₃ ) having a thickness of 30 nm to 200 μm.

[0011]

These conventional techniques for forming a stable rust layer itself by surface treatment or heat treatment of steel materials are
This technology is noteworthy in terms of the components and composition itself of the stable rust layer. That is, in the weathering steel and oxide dispersion steel, etc., containing many additive elements, weldability during steel material construction, melting, and efficiency during steel material production such as rolling are inevitably higher than ordinary steel. Decrease. In addition, the production cost is lowered due to the reduction of production efficiency and the addition of many additive elements, and the welding cost is also reduced due to the reduced weldability. Therefore, without using these weather resistant steels,
If high corrosion resistance of steel can be realized by using components and composition of stable rust layer using ordinary carbon steel or low alloy steel, there are many advantages in terms of production efficiency and cost, or construction efficiency and cost. There is.

However, the rust formed by α-FeOOH containing 0.3 wt% or more of Cr, Cu, P and Ni as described in JP-A-06-93467 and the above-mentioned JP-A-09-93467. Rust consisting of hematite (αFe ₂ O ₃ ) as in JP 125224 deteriorates the coating film when it is used without coating (bare use) in the salt corrosive environment or when it is used after painting. The present inventor has found that high corrosion resistance is not always exerted when it is destroyed. Then, as one of the causes, it was also found that the method for producing each of the above-mentioned films (rust) is not easy, and even if chemical treatment or heat treatment for production is performed, reproducibility is not high and high corrosion resistance is not exhibited.

Therefore, in view of the problems of these conventional techniques of forming a stable rust layer itself by surface treatment or heat treatment of steel materials, even ordinary carbon steel or low alloy steel is not coated or not coated. An object of the present invention is to provide a method for forming rust on a steel material, which is suitable for a structural material used for painting and has excellent reproducibility and corrosion resistance.

[0014]

The gist of the present invention for this purpose is that Ti, Nb, Ta,
Contacting a solution containing one or more of Zr, V, Hf ions and / or acid ions of these elements,
After that, on the rust layer on the steel surface or steel surface, Ti, Nb, Ta,
This is to generate rust containing one or more of Zr, V, and Hf in a total amount of 0.05 wt% or more.

By adopting the above-mentioned gist, the rust generated on the surface of the steel material during use as a structure can be made into a dense and stable rust layer, and reproducible and high corrosion resistance can be obtained even in a salt corrosion environment. It becomes possible to have.

The present inventors have examined the relationship between the components and composition of rust formed on the surface of steel materials and the corrosion resistance in a salt corrosion environment. As a result, on the steel surface or steel rust layer, Ti,
It was found that high corrosion resistance in a salt corrosion environment can be reproducibly exhibited by containing or existing one or more of Nb, Ta, Zr, V, and Hf.

That is, Ti, Nb,
If one or more of Ta, Zr, V, and Hf are contained or present, the rust generated in the atmospheric environment on the steel surface or steel rust layer after that contains these elements, resulting in salt corrosion environment. Even underneath, it forms as fine and dense α-FeOOH rust and amorphous rust, and in this process β-
We have found that the generation of FeOOH is suppressed as much as possible.

The action of Ti and the like on rust formation is as follows.
There is still room for elucidation regarding the composition (structure) of rust. However, the composition of rust excellent in corrosion resistance including Ti and the like is obtained by the X-ray diffraction method. As a result, fine and dense α-FeOOH rust and amorphous rust are formed on the steel surface or the steel rust layer. At the same time, the generation of β-FeOOH is suppressed as much as possible,
And, in this rust, the fraction of α-FeOOH component and amorphous component is preferably 35 wt% or more, β-FeOOH
It has been found that rust having a component fraction of preferably 20 wt% or less can exhibit high corrosion resistance in a salt corrosive environment, particularly with good reproducibility.

Indeed, as disclosed in the above-mentioned Japanese Patent Laid-Open No. 06-93467, it is preferable that the component of the rust layer formed on the surface of the steel material is dense α-FeOOH. However, in order to exhibit high corrosion resistance with good reproducibility, it is considered necessary that β-FeOOH is not present in this dense rust, in other words, it is necessary to suppress the formation of β-FeOOH as much as possible.

Therefore, even in the above-mentioned Japanese Patent Application Laid-Open No. 06-93467, although attention is paid to α-FeOOH rust, when a steel material is used in the above salt corrosive environment or when it is used as a coating, a coating film is formed. It is considered that the reason why high corrosion resistance is not always exhibited when the alloy is deteriorated or destroyed is due to the generation of β-FeOOH in the rust, which is not noted in this publication.

That is, no matter how high the content ratio of amorphous or α-FeOOH in rust, β-FeOO is particularly apt to promote corrosion.
When H is present, this β-FeOOH serves as the starting point and corrosion progresses. And this phenomenon becomes remarkable especially in a salt corrosive environment. Therefore, how to suppress this β-FeOOH is the key to whether the stable rust layer exhibits high corrosion resistance.

In this respect, one or two or more of Ti, Nb, Ta, Zr, V, and Hf, in particular, Ti is contained in rust in an amount of 0.05 wt% or more, preferably 0.1 wt% or more. Amorphous rust generated during use and the proportion of α-FeOOH are increased to form fine and dense rust, and a stable rust layer in which β-FeOOH is suppressed is formed. As a result, high corrosion resistance is exhibited particularly in a salt corrosive environment.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION First, the meanings of Ti, Nb, Ta, Zr, V and Hf contained in rust in the present invention will be described in detail. One or more of Ti, Nb, Ta, Zr, V and Hf,
By containing 0.1 wt% or more in rust, the proportion of amorphous rust and α-FeOOH generated during the use of steel materials is increased to form fine and dense rust and suppress β-FeOOH. A stable rust layer can be formed, and as a result, high corrosion resistance is exhibited especially in a salt corrosion environment. The denser the rust formed during the use of the steel material, the higher the effect of preventing the entry of corrosion factors such as chloride ions.

The mechanism of the effect of forming fine and dense rust of Ti, Nb, Ta, Zr, V and Hf and the effect of suppressing β-FeOOH has not been clarified yet. However, they form fine carbide or nitride precipitates in rust and / or form minute colloids or hydroxides when the steel dissolves when rust occurs, and the rust generated by these forms. Amorphization and α-FeOO
It is considered to be the core of H conversion. On the other hand, it is considered that the existence of nuclei such as those mentioned above conversely inhibits and suppresses the generation of crystalline rust such as β-FeOOH, which is rough and brittle and easily peels off.

The effect of these Ti, Nb, Ta, Zr, V, and Hf is that the total (total amount) of one or more of these elements is 0.05 wt% or more, preferably 0.1 wt% or more in rust. It is exhibited by containing it, and if the content is less than 0.05 wt%,
This effect is not exerted. However, even if the content exceeds 50 wt%, the effect is the same, and there is a possibility that the corrosion resistance is rather deteriorated, for example, the adhesion between rust and the steel surface is reduced. Therefore, the upper limit is 50wt% in total.
It is preferably about the same. Among these elements, Ti has the highest effect of improving the corrosion resistance as described later. Therefore, when these elements are contained alone or in combination in rust, it is preferable that Ti is essential. Further, without containing Ti, when containing one or more of Nb, Ta, Zr, V, and Hf in rust, it may be contained in an amount larger than the content when Ti is set as a reference, It is preferable for surely exhibiting the effect of corrosion resistance.

Further, rust Ti, Nb, Ta, Zr, V of the present invention,
Regarding the contained elements other than Hf, the inclusion of other elements as impurities is acceptable within a range that does not inhibit the effects of these elements or the formation of rust intended by the present invention. Among these, one or more of Cr, Cu, P, and Ni disclosed in JP-A-06-93467 may be contained as the other element. As described above, although these elements alone cannot reliably improve the corrosion resistance of rust,
When used in combination with elements of Ti, Nb, Ta, Zr, V, and Hf in rust in an amount of 0.3 wt% or more, it has a composite effect that contributes to amorphization of rust and formation of α-FeOOH. Can be expected.

Next, the components and composition of rust in the present invention will be described below. In the present invention, the main component of rust is α-Fe.
It is preferably made of OOH or amorphous rust. Normally, the main component of iron rust formed on the surface of steel is α
-FeOOH, β-FeOOH, γ-FeOOH and Fe ₃ O ₄ are crystalline rust and amorphous rust. Among them, amorphous rust forms a finer and more stable stable rust layer than crystalline rust. Moreover, even if a "defective portion" as a rust film is formed due to crystalline rust during the use of steel, the amorphous rust portion fills this hole and reduces the "defective portion". It also has a repair function. As a result, the long-term bare weatherability of the steel material is guaranteed. Therefore, the higher the proportion of amorphous rust in the iron rust (amorphous degree) and the higher the proportion of fine and dense α-FeOOH among the crystalline rust components, the higher the corrosion resistance. Further, when the steel material is further coated and used, these dense rusts provide good adhesion with the coating film and guarantee the long-term corrosion resistance of the steel material. Therefore, in the present invention, as a preferable condition, the fraction of the amorphous component of the rust formed on the surface of the steel material, which is obtained by the X-ray diffraction method, is specified to be 35 wt% or more.

On the other hand, rust other than this, particularly crystalline rust such as β-FeOOH, progresses corrosion starting from this rust even if the ratio of the amorphous or α-FeOOH in the rust is high. Therefore, it is necessary to suppress it as much as possible. Therefore, in the present invention,
As a preferable condition, the proportion of β-FeOOH component of the rust formed on the surface of the steel material, which is determined by the X-ray diffraction method, is restricted to 20 wt% or less. The amorphous component of rust and the fraction of α-FeOOH are 35
If it is less than wt% and the fraction of the β-FeOOH component exceeds 20 wt%, the β-FeOOH, γ-FeOOH and Fe ₃ O _{4 are} added.
Since the amount of the crystalline rust component is increased and the rust on the surface of the steel material does not form a dense stable rust layer, it may not be possible to guarantee the high corrosion resistance of the steel material.

In the present invention, the high corrosion resistance of the rust formed on the surface of the steel material means the corrosion resistance of the steel material in a salt corrosion environment. Therefore, in order to guarantee this high corrosion resistance, evaluate the steel corrosion resistance after atmospheric exposure including atmospheric exposure, including salt water spray simulating a salt corrosive environment (5% salt water spray once a week). There is a need.

As a means for measuring the amorphousness of rust, quantification of iron rust component by powder X-ray diffraction method and its quantification in "Corrosion protection 95 C-306 (pp. 341-344)" The powder X-ray diffraction method disclosed in “Application” is effective. In this document, the powder X-ray diffraction method was applied to weather-resistant steel materials, and an attempt was made to quantify the iron rust component on the steel surface.The higher the proportion of amorphous rust in iron rust (amorphous degree), It supports the corrosion resistance improvement model that becomes a precise and stable rust layer. As a more specific powder X-ray diffraction method, in the same literature, a standard X-ray diffraction method was used in which a fixed weight ratio of CaF ₂ or ZnO as an internal standard was mixed with a rust sample taken from steel and powdered. The rust component of each crystallinity was quantified from the integrated intensity ratio of the unique diffraction peaks of each of the above 5 types of rust and the calibration curve of each rust component obtained in advance, and the total rust The ratio of the amorphous component is calculated by subtracting the amount of each crystalline rust component from the amount. This is because it is difficult to obtain the integrated intensity ratio of the diffraction peak of the amorphous component itself and it is difficult to quantify it.

Incidentally, as disclosed in the document,
In addition to X-ray diffractometry, other analytical methods such as infrared spectroscopic analysis can perform qualitative analysis of rust components, but quantitative analysis is difficult. There is no. Therefore, the amorphousness of rust on the surface of steel referred to in the present invention means the X-ray diffraction method, especially the powder disclosed in the above-mentioned document.
It is measured quantitatively by the X-ray diffraction method.

Next, the method of the present invention for forming these dense stable rust layers will be described. First, an appropriate treatment such as cleaning, cleaning, or surface polishing is performed on the surface of the steel material before or during use as a structural material. These treatments include things such as mirror-finishing the steel surface, removing rust on the steel surface, or simply cleaning the steel surface or steel rust layer.
This may be selectively performed depending on the surface condition required for the steel material. Also, without removing the steel rust layer, against existing rust,
The forming method of the present invention may be carried out. In that case, the steel material rust layer may be simply cleaned or may not be cleaned.

Next, in the present invention, one or more of Ti, Nb, Ta, Zr, V, and Hf ions and / or acid ions of these elements are added to the steel material surface or the rust layer on the steel material surface. A solution such as an aqueous solution or an organic solvent contained is brought into contact. At this time, it is preferable that the solution essentially contains Ti ions or Ti acid ions because of the superiority in the effect of forming the dense rust of Ti as compared with other elements. When these ions or acid ions of these elements are contained, it is preferable to use sulfates or chlorides of these elements from the viewpoint of solution stability and throwing power.

The most convenient method for bringing the solution into contact with the surface of the steel material or the rust layer on the surface of the steel material is to apply the solution to the steel material. However, an ordinary solution treatment method such as immersing a steel material in a solution is appropriately selected according to circumstances and circumstances.

Further, when one or more of Cr, Cu, P and Ni are contained in rust in an amount of 0.3 wt% or more, the above Ti,
A solution containing Nb, Ta, Zr, V, and Hf is added with Cr, Ni, and C.
One or more u or P ions or acid ions are contained. The introduction of these Cr, Cu, P, and Ni is performed using Ti, Nb, T
Although it may be possible to perform it in a solution different from the solution containing a, Zr, V 2, and Hf, or in a separate process, the same treatment solution can be used more easily. And, of these, it is preferable to introduce Cr, which has a particularly high effect of forming dense rust. When these ions or acid ions of these elements are contained, it is preferable to use sulfates or chlorides of these elements from the viewpoint of solution stability and throwing power.

A steel material in which Ti, Nb, Ta, Zr, V, and Hf are present on the surface of the steel material is not required to be positively treated.
Further, even under a salt corrosive environment such as salt water or snow-melting salt coming in, it is a great advantage that a dense stable rust layer is formed in a relatively short time during use as a structural material such as a bridge. However, from the viewpoint of quality assurance that guarantees reliable corrosion resistance such as bare weather resistance, after the steel material is manufactured, if necessary, pretreatment such as pickling is performed, and then heat treatment is performed in an atmosphere such as a gas with a controlled oxidation potential. Alternatively, chemical surface treatment with chemicals such as phosphates, chromates and oxidizers is performed to amorphize the rust that is generated during the steel manufacturing process, to make it more aggressive and precise. A stable rust layer may be formed.

Further, the present invention can be used not only for a steel material for a new structure but also for improving the corrosion resistance of a coated or unpainted steel material used as an existing structure. That is, the coating film or rust on the surface of the steel material in use as an existing structure is completely or partially (for example, only the corroded part) peeled or cleaned without peeling, and the solution of the present invention is applied to the steel surface or the steel material. Even if it is applied to the rust layer, it is possible to generate fine rust over time. Therefore, the present invention can also be used as repair or maintenance of existing structures.

Regarding treatment methods other than the chemical treatment method of the present invention, a vapor phase coating method for concentrating or presenting these elements on the surface of the steel material by sputtering, vapor deposition or the like, or containing these elements and for treating these elements on the surface By the method of using the steel material concentrated to, the base material to form the dense stable rust layer of the present invention, Ti, Nb, Ta, Zr,
There is a method of forming a steel material surface in which V 2 and Hf are present.

However, it is difficult to increase the amount of these elements and the amount of surface enrichment of the steel material by the method of migrating the elements contained in the steel material into the rust by diffusion or the like. There is a risk of obstructing other properties such as properties. Furthermore, even if the amount of elements or the amount of surface concentration is increased, it is difficult to include a predetermined amount (above the lower limit amount) of elements in rust. And also, the advantage of being able to use normal carbon steel or low alloy steel may be lost.

Further, in the vapor phase coating method, the equipment and the processing cost itself are expensive, and the processing of a large amount of a large thick steel plate is not efficient itself, and the feasibility is poor.

Further, the steel itself used in the present invention does not include high alloy steel such as stainless steel that does not cause rust, and basically, ordinary low carbon steel or low alloy steel can be used. Among these, the weather resistant steel containing the conventional P.sub.2, Cu, Cr, Ni, Ti, etc. may be used under more strict corrosion resistance specifications and operating environments.

Further, since the steel material of the present invention is for structural materials such as the above-mentioned minority main girder bridge, from the viewpoint of workability and shortening of the construction period, the heat input amount of 5 KJ / mm or more by carbon dioxide gas arc welding or electrogas arc welding. In some cases, large heat input welding with heat input of 100 to 300 KJ / mm or more is applied. Therefore,
As the steel material used for this structural material, not only mechanical properties such as strength as a structural material, but also preheating is not required, and excellent weldability capable of high efficiency welding such as large heat input welding is provided. A steel material having both corrosion resistance is preferable.

It should be noted that, under more severe specifications for corrosion resistance and environments where salt corrosion is used, it is necessary to pay attention to the elements that hinder the stable formation of rust in the composition of the steel. That is, S and Cr are examples of elements that hinder the formation of stable rust.

If the content of S exceeds 0.02%, the formation of the stable rust layer due to the inclusion of Ti or the like in rust may be impaired, leading to deterioration of corrosion resistance. Therefore, the S content is preferably 0.02% or less.

In the conventional weathering steel material, Cr is P, Cu or Ni.
At the same time, it is recognized as an essential additional element for forming the stable rust layer.
~ 1.25% content. In addition, the above-mentioned JP-A-58-25458
In Japanese Patent No. 2572447 and Japanese Patent No. 2572447, the addition of Cr is not specified, but it is inevitably contained in an amount of 0.05% or more as an impurity from the iron raw material or the steelmaking process.

However, when Cr is contained in an amount of 0.05% or more, even if slight corrosion starts in the microscopic surface defects of the steel, Cr ions dissolved in a trace amount with the iron atoms in a chemical equilibrium also add the action of Cl ions. It causes a decrease in pH in the microscopic surface defect portion of the steel, has an action of promoting the oxidizability of condensed water in the defect and inducing corrosion. Therefore, even if the dense stable rust layer is generated, Cr has an action of promoting the corrosion of steel in the lower part of the stable rust layer, and inhibits the adhesion between the rust layer and the steel, resulting in the peeling of the rust layer. May be promoted or, as a result, the generation or maintenance of a dense stable rust layer may be hindered. Therefore, it is preferable to reduce the Cr content as much as possible, and it is preferable to set the upper limit to less than 0.05% in consideration of the economical efficiency of reducing the Cr content.

On the other hand, it is preferable to contain Ti in steel as an element for promoting the formation of a stable rust layer in place of Cr. As mentioned above, the Ti contained in the steel is the only Ti in the rust.
Although it is difficult to secure or guarantee the
It does not cause a decrease in the pH like Cr, but has the effect of promoting the formation of the stable rust layer, and has the unique property of synergistically increasing the effect of promoting the formation of the stable rust layer of Ti in the rust layer. .
Specifically, while increasing the proportion of amorphous in the iron rust and the proportion of rust of α-FeOOH, suppress the formation of β-FeOOH which is particularly easy to promote corrosion among crystalline rust components, Promotes the formation of a fine and precise stable rust layer. As a result, the penetration of corrosion factors such as chloride ions into the rust layer is prevented, the dense stable rust layer is maintained, and the corrosion resistance is improved. Ti content is 0.
If it is less than 01%, this effect does not exist, and if it exceeds 1.0%, the effect is saturated and it is not economical. In this respect, in order to exert the effect of Ti more, it is preferable to contain Ti in the steel in an amount of 0.05% or more, and if Ti in the steel exceeds 0.5%, embrittlement of the steel may become a problem. , Not economical. Therefore, when Ti is contained in the steel, the preferable Ti content is 0.
It is in the range of 05 to 0.5%.

S in steel that inhibits the formation of this stable rust
As a steel containing Ti having the effect of promoting the formation of a stable rust layer while controlling Cr, the present inventors previously disclosed, as Japanese Patent Application No. 09-330173, C: 0.15% or less, Si: 0.10 ~
1.0%, Mn: 1.5% or less, S: 0.02% or less, P: 0.05%
Below, Cr: 0.05% or less, Ti: 0.01 to 1.0%, Ca: 0.0001
~ 0.01% and Cu: 0.05-3.0% and Ni: 0.05-6.0%
A steel material containing one or two kinds and having a basic composition consisting of the balance Fe and inevitable impurities was filed. This steel material also has good weldability and is the most suitable as a preferred embodiment of the steel material used in the present invention.

In addition to this basic composition, the following elements other than Ti having the effect of promoting the formation of a stable rust layer may be included in the steel. Mo: 0.05-3.0% and W: 0.05-3.0% 1
Seed or two species. Al: 0.05 to 0.50%, La: 0.0001 to 0.05
%, Ce: 0.0001 to 0.05%, Mg 0.0001 to 0.05%, 1 or 2 or more. 0.50% or less in total of one or more of Zr, Ta, Nb, V and Hf.

Next, a method for manufacturing the steel material of the present invention will be described.
The steel material of the present invention can be manufactured by a normal method for manufacturing a thick steel plate having a thickness of 50 mm or more. That is, after steel is continuously cast or melted by an ingot making method, slab rolling, hot forging, or hot working such as thick plate rolling is performed to manufacture a predetermined product sheet thickness. The hot working conditions and the cooling and heat treatment conditions after the hot working are 390 to 90% for steel structural materials such as bridge structural materials.
630N / mm ² Grade or higher is determined as appropriate according to requirements and specifications of mechanical properties such as strength. Therefore, in addition to normal hot working, after ensuring low alloying or low carbon equivalent for ensuring weldability, mechanical properties such as the strength are secured, and the steel material structure of the present invention is described above. Preferably, in order to set the ferrite content to 90% or more, forced cooling such as accelerated cooling after hot working or controlled rolling may be performed. Further, the heat treatment after the hot working may be appropriately performed such as rolling online direct quenching (DQ) or offline quenching and tempering (QT).

[0051]

EXAMPLES Next, the significance of each requirement of the method for forming a steel material rust according to the present invention described above will be described with reference to examples. table
Each steel ingot having the chemical composition shown in 1 was melted, and these steel ingots were hot-rolled and then forcedly cooled by accelerated cooling to obtain a plate thickness of 50 mm.
Thick steel plate was manufactured. No. 1 in Table 1 is low carbon steel, No. 2 is Ti
Corrosion-resistant low-alloy steel, No.3, is a weather-resistant steel with added Cr and reduced Cr. Then, test pieces were cut out from these thick steel plates, the surface of the test piece was mirror-finished by emery paper polishing and buffing, and Ti, Nb, Ta, Z
A treatment was performed by applying an aqueous solution containing sulfates of r, V, and Hf, and an aqueous solution containing sulfates of Cr, Ni, Cu, and P in addition to these to the surface of the steel material.

The test pieces that were subjected to these treatments were the same as the invention examples Nos. 2 to 11 in Table 1 as they were, and the invention examples Nos. 13 to 16 in Table 1 were used. Is a phthalate resin that is commonly used for painting bridges, etc.
A corrosion resistance test was performed as a 50 μm coated test piece.
Corrosion resistance test of bare test piece is conducted by atmospheric exposure test, 5% salt water is sprayed once a week, simulating an actual salt corrosion environment, and the test piece is facing south and at 30 ° to the horizontal. It was installed at an inclination of, and an atmospheric exposure test was carried out for 15 months, and long-term durability was evaluated by measuring the reduction amount of average plate thickness (corrosion reduction amount).
The average thickness reduction amount was obtained by measuring the average thickness of the test material before and after the atmospheric exposure test with a micrometer and calculating the average thickness reduction amount (mm) in consideration of the density.

Among the relatively short-term corrosion acceleration tests such as the salt spray test, the purpose of carrying out the atmospheric exposure test for one year was that the steel of the present invention was used especially for bridges under a salt corrosion environment. Because it is a structural material such as, the accurate evaluation cannot be performed unless the test is suitable for the corrosion under the actual use condition.

As for the coating test piece, the coating film was scratched in advance to provide an artificial coating film defect, and an atmospheric exposure test was conducted under the same conditions as those of the bare test piece, and the artificial coating film defect portion after the test was performed. The corrosion resistance was evaluated by measuring the blister width of the. Then, from these results, comprehensive evaluation of corrosion resistance (◎ ○ △ ×)
I went. The results are shown in Table 2. In Table 2, the average plate thickness reduction (corrosion reduction) is shown in mm.
The bulge width of the artificial coating film defect part is A for 0.80 mm or more, 0.5 ~
0.8 mm is described as B, and 0.5 mm or less is described as C.

Further, after the air exposure test, the elements and the amount of elements in the rust formed on the surface of the test piece were analyzed and measured by the X-ray diffraction method (XR
D) and electron probe X-ray microanalysis (EPM
A) and the composition of rust was analyzed by the X-ray diffraction method. More specifically, the above-mentioned "corrosion protection 95 C-
306 (pp. 341-344) '', and a powdered mixture of rust samples taken from steel with a certain weight ratio of ZnO as an internal standard was identified by the X-ray diffraction method. , Α-FeOOH, β-FeOOH, γ-FeOOH and Fe ₃ O ₄ integrated intensity ratio of the unique diffraction peak of each of the five types of crystalline rust, from the calibration curve of each rust component obtained in advance, The quantification of each crystalline rust component was performed. And
The ratio (%) of the amorphous component was calculated by subtracting the amount of each crystalline rust component from the total amount of rust. These results are also shown in Table 2.

In Table 2, the composition of the generated rust on the surface of the test piece after the atmospheric exposure test was as follows: α-FeOOH and amorphous rust component fractions A: 0-35 wt%, B: 35-40 wt% , C: 40 wt% or more, β-FeOOH rust component fraction A: 31 wt% or more, B: 20-30
wt%, C: less than 20 wt%.

For comparison, the test steels shown in Table 1 were subjected to a corrosion resistance test with bare test pieces in the same manner as the invention examples without the treatment according to the present invention. Without performing the treatment according to the invention, Comparative Example No. 12 was also subjected to a corrosion resistance test in the same manner as the invention example, also analyzed and evaluated in the same manner as the invention example,
The results are shown in Table 2.

As is clear from the results shown in Table 2, Invention Examples Nos. 2 to 15 satisfying the conditions of the present invention have excellent corrosion resistance both in the case of unpainted and in the case of unpainted. Incidentally, in the comparison between the invention examples, Invention Example No. 4 using Ti sulfate, compared with Invention Example No. 5 treated with chloride Ti, is excellent in corrosion resistance, using a sulfate salt. This proves that is superior in corrosion resistance. Further, invention example N having a low content of Ti and the like
o.2, 3, 13 are inferior in corrosion resistance to other invention examples,
The importance of the content of these elements in rust is supported.

On the other hand, without performing the processing according to the present invention,
Comparative example N in which corrosion resistance test was performed on bare test pieces as in the invention example
o.1, Comparative Example No. 12 in which the corrosion resistance test was performed on the coating test piece in the same manner as the invention example without the treatment according to the present invention, all had a corrosion weight loss of 0.80 mm or more and a coating film swelling width was A of 0.80 mm or more is large, and the corrosion resistance is extremely poor. In these comparative examples, the rust composition is a structure in which α-FeOOH and an amorphous rust component are the main components, but Ti, Nb, Ta, Zr, V, or Hf is not contained in the rust or the content is Since the amount of β-FeOOH is small, the ratio of crystalline rust of β-FeOOH increases.
The corrosion resistance is inferior because the corrosion progresses starting from. Therefore, from this result, the amorphization of rust on the steel surface, which is a more preferable condition of the present invention, and β-Fe
Ti, Nb, Ta, Zr, V to suppress the crystalline rust of OOH
The necessity of inclusion of Hf in rust is understood.

As a result of measuring the degree of concentration of chloride ions at the interface between the rust layer on the surface of the test piece and the base metal by the EPMA method, all of the invention examples showed that the concentration of chloride ions at the interface between the rust layer and the base metal was concentrated. In contrast, in Comparative Example, the chloride ion concentration at the interface between the rust layer and the base iron was large, and the results of the corrosion resistance test were supported.

[0061]

[Table 1]

[0062]

[Table 2]

[0063]

Industrial Applicability According to the present invention, it is possible to provide a steel material having excellent corrosion resistance especially in a salt corrosive environment. Therefore, in particular, new applications of this type of steel with excellent corrosion resistance,
Moreover, it will be greatly expanded and has great industrial value.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-10-183364 (JP, A) JP-A-53-5039 (JP, A) JP-A-6-136557 (JP, A) JP-A-5- 247663 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C23C 22/00-22/86

Claims (8)

(57) [Claims] 1. A steel material surface or a rust layer on a steel material surface is provided with T
i, Nb, Ta, Zr, V, Hf ions and / or contact with a solution containing one or more kinds of acid ions of these elements, and then the rust layer on the steel surface or steel surface,
One or more of Ti, Nb, Ta, Zr, V, and Hf in total
A method for forming steel rust excellent in corrosion resistance, characterized in that rust containing 0.05 wt% or more is generated. 2. The method for forming a steel rust excellent in corrosion resistance according to claim 1, wherein the solution essentially contains Ti ions or Ti acid ions, and Ti is essentially contained in the produced rust. 3. The method for forming steel rust excellent in corrosion resistance according to claim 2, wherein the solution essentially contains Ti sulfate. 4. The solution further contains one or more kinds of Cr, Ni, Cu, P ions or acid ions, and one or more kinds of Cr, Ni, Cu, P 2 in the generated rust. 0.3wt%
The method for forming steel rust excellent in corrosion resistance according to any one of claims 1 to 3, which is contained as described above. 5. The method for forming steel rust excellent in corrosion resistance according to claim 4, wherein the solution further essentially contains Cr sulfate. 6. The fraction of α-FeOOH component and amorphous component of the formed rust obtained by X-ray diffraction method is 35 wt% or more, and the fraction of β-FeOOH component is 20 wt% or less. 1
6. A method for forming steel rust excellent in corrosion resistance according to any one of items 1 to 5. 7. The method of forming a steel material rust excellent in corrosion resistance according to claim 1, wherein the method of forming a steel material rust is used for repairing a steel material used in a building. 8. The method for forming steel rust excellent in corrosion resistance according to claim 1, further comprising forming a coating film on the surface of the steel material contacted with the solution.